1. Field of the Invention
The present invention relates to a delay fluctuation absorbing method of absorbing fluctuation of cell transmission delay which is generated at different levels for different cells depending on a traffic situation in the network of a cell transmission system when a cell containing encoded data arrives at a receiver side through the network, and an exchange which utilizes this method.
2. Description of the Related Art
In a cell transmission system, data to be transmitted is encoded and assembled into a cell, and the cell is transmitted to a receiver side through a network. In such a cell transmission system, in order to achieve a high decoding efficiency at the receiver side, so-called delay fluctuation absorbing control is performed to absorb the fluctuation of cell transmission delay so that each cell can make its reproduction timing at the receiver side.
As one of the systems to which the delay fluctuation absorbing control is applied, there is known a system which handles voice cells created by encoding voice signals at a variable rate.
FIGS. 7(a)-7(f) are timing charts illustrating an example of the time stamping operation during cell transmission in a cell transmission system in which the variable rate voice encoding is performed.
To describe the time stamping operation, the operation is divided into an operation at the sender side (FIG. 7(a) to (c)) and an operation at the receiver side (FIG. 7(d) to (f)).
In the operation at the sender side, a voice signal (FIG. 7(a)) is sampled and encoded to obtain coded data (FIG. 7(b)).
Then, cells are assembled for compressed data of PCM sampled data for the number of units (reference interval in FIG. 7(b)) which changes for each encoding rate (variable rate) while changing the encoding rate depending on the characteristics of voice data to obtain a transmission cell (FIG. 7(c)) of fixed length.
At this time, a control header is added to the cells for each cell unit as a control data for use in cell sequence control or discarded cell detection at the receiver side.
In this example, time stamp number (TS) in modulo 32 is added as the control header to maintain the reproduction timing.
In setting the time stamp number, the time stamp number at the time of starting the encoding operation is held and attached at the end of the encoding operation. By setting the time stamp number in this manner, decoding time can be secured at the time of decoding.
Further, a sequence number (SN) in modulo 8 is attached as a control header for detection of a discarded cell, and cell transmission is carried out each time the encoding operation is completed.
The time stamp number and sequence number in the control header are different in that the time stamp number is a non-consecutive number determined according to the encoding rate whereas the sequence number is a consecutive number.
On the receiver side, the cell transmitted from the sender side through the network is received with a delay (within-network delay (d.sub.CIR))) (FIG. 7(d)).
To prevent the subsequent cells from being discarded due to delay fluctuation, i.e., delay varies for each cell, fluctuation absorbing time (d.sub.Y) is provided. During the fluctuation absorbing time, received cells are stored in a buffer and, after passage of the delay fluctuation time d.sub.Y after the reception of the heading cell, a reproduction timing number is generated with the same period as in the time stamp number of the cell. The current reproduction timing number is compared with the time stamp number of the received cell. When coincidence is found between the time stamp number and the current timing number, the cell is reproduced (FIG. 7(e)).
Through the above reproducing operation, substantially the same reproduction voice waveform (FIG. 7(f)) as the voice waveform sent from the sender side can be obtained.
In such a conventional delay fluctuation absorbing method for the variable rate voice encoding, as the delay fluctuation absorbing time (d.sub.Y), a constant time (d.sub.X %) which guarantees more than a cell discard allowance probability corresponding to an X % delay is previously allocated on an end-to-end basis at the time of path setting.
Generally, in this type of cell transmission system, relationship between the number of received cells and the delay fluctuation time is generally considered to have such a distribution as shown in FIG. 8.
In the cell distribution, a delay corresponding to a partition point of the X % cell region (shadowed area) is referred to as X % delay, and a fluctuation absorbing time capable of absorbing the X % delay to guarantee a maximum cell reception probability is referred to as the fluctuation absorbing time d.sub.X % for the X % delay.
In actual operations for this type of cell transmission system, within-network delay fluctuates for each cell vary depending on current situations in the network.
Accordingly, the conventional control method for setting the fixed delay fluctuation absorbing time (d.sub.X %) has a problem that, when different cells provide different delays due to traffic situation or the like, the fluctuation absorbing time tend to be too large or too small, whereby the quality of voice to be reproduced varies.
In particular, in a system in which cell transmission is carried out based on the variable encoding rate, different cells provide different encoding rates, whereby the decoding time is not always the same for each cell. (The cell generation interval is not constant.)
That is, in such variable encoding rate-system, in addition to the problem of different within-network delays caused by different cells, there is another problem that encoding time differs for each cell.
As described above, in the case where different cells provide different encoding rates and thus provide different decoding times, when a fixed fluctuation absorbing time is set, there occurs such a problem that the fluctuation absorbing time becomes larger or smaller than a desired value, which results in that cell discard rate exceeds the X % delay allowance discard rate.